1. Field of the Invention
The present invention relates to a probe and a near-field microscope capable of obtaining information, such as dielectric properties, of an analyte using electromagnetic waves. More specifically, the present invention relates to a probe and a near-field microscope capable of observing physical properties, such as dielectric properties, of a minute region of an analyte using electromagnetic waves in the frequency region from the millimeter waveband to the terahertz band (30 GHz to 30 THz) (hereinafter also referred to as “high-frequency electrical signal”).
2. Description of the Related Art
As a method for observing dielectric properties of a minute region of an analyte or a minute analyte, there is known a method in which the reflection of a high-frequency electrical signal from an analyte is measured using a high-frequency transmission line, such as a coaxial line or a high-frequency co-planar strip line, with a minute tip. This method is used in a wide frequency region from the microwave region to the visible region, and is called microwave probe (or microwave probe microscope) in the microwave region. Since a microwave probe uses a minute structure smaller than the wavelength, dielectric properties of a region smaller than the wavelength of the high-frequency electrical signal to be used can be measured. Therefore, the distribution of dielectric properties of an analyte can be imaged with high spatial resolution. In addition, the information of dielectric properties of an analyte can be read from the phase retardation and the decrease in amplitude of the high-frequency electrical signal reflected from the analyte. The phase retardation reflects the real part of the dielectric constant of the analyte, and the decrease in amplitude reflects the imaginary part of the dielectric constant of the analyte. Therefore, by analyzing these, dielectric properties of the analyte can be quantitatively evaluated.
Japanese Patent Laid-Open No. 2005-121422 discloses an apparatus that measures the complex dielectric constant of an analyte with a microwave probe according to the above-described principle. In the configuration of Japanese Patent Laid-Open No. 2005-121422, a generator transmits a high-frequency electrical signal to a coaxial line with a minute tip, and a reflected wave from an analyte is received by a detector via a directional coupler. In addition, it is also possible to provide a resonance structure of the high-frequency electrical signal in a high-frequency transmission line so as to make a high-frequency signal reflected from the analyte a standing wave, and to use a shift of resonance frequency or a change in resonator Q-value with the change in dielectric properties of the analyte, for imaging.
Japanese Patent Laid-Open No. 2002-189043 also discloses an apparatus that measures the complex dielectric constant of an analyte with a microwave probe according to the above-described principle. In the configuration of Japanese Patent Laid-Open No. 2002-189043, a generator transmits a high-frequency electrical signal to a multi-conductor transmission line with a minute tip via a first coupling probe, and a reflected wave from an analyte is received by a detector via a second coupling probe. By appropriately terminating the tip and the opposite end of the multi-conductor transmission line, the reflected wave from the analyte is made a standing wave in the multi-conductor transmission line.
The spatial resolution of these microwave probes is 1/1000 or less of the wavelength of the microwave. Therefore, they are so-called near-field probes (or near-field probe microscopes) in the visible region.
There are a variety of high-frequency transmission lines. Kanglin Wang, Daniel M. Mittleman: Nature, vol. 432 (2004) discloses a wire waveguide consisting of a single conductor. The wire waveguide features a capability of transmission of a high-frequency electrical signal across a comparatively wide frequency band. For example, it is known that its propagation loss is smaller than those of other multi-conductor high-frequency transmission lines in the frequency region from the millimeter waveband to the terahertz band, and its dispersion is also comparatively small.
However, in the microwave probe of the Japanese Patent Laid-Open No. 2005-121422, it is necessary to use a directional coupler or the like in order to branch the reflected wave of the high-frequency electrical signal from the analyte. This complicates the apparatus. In addition, in the frequency region from the millimeter waveband to the terahertz band, when the frequency of the high-frequency electrical signal to be used is high, the propagation loss in the high-frequency transmission line is not negligible. In the multi-conductor high-frequency transmission line as in Japanese Patent Laid-Open No. 2002-189043, the propagation loss is comparatively great, and therefore the sensitivity is low. Kanglin Wang, Daniel M. Mittleman: Nature, vol. 432 (2004) discloses only a wire waveguide consisting of a single conductor.